1. Field of the Invention
The present invention relates to a preform for a plastic optical material, a producing method thereof, an optical coupling method of a plastic optical fiber used in an optical networking and the like, and a connector used for the optical coupling.
2. Description Related to the Prior Art
As having merits in easiness of processing and production, low price and the like, plastic optical member, compared with an optical member of quartz having the same structure, is recently utilized for several members, such as an optical fiber, an optical lens, an optical waveguide and the like. Especially the plastic optical fiber among these members is entirely made of plastic. Therefore the plastic optical fiber has merits in adequate flexibility, smaller weight and more easy treatment and production as the optical fiber having large diameter, and the lower production cost than the quartz optical fiber, although having demerit in larger transmission loss than the optical member of quartz. In Japanese Patent Laid-Open Publication No. S61-130904, it is planned to use several sorts of the plastic optical fibers as transmitting mediums for such a short distance transmission that the transmission loss is not so large to an influence on the transmission.
The plastic optical fiber is constructed of a core whose main component is organic compounds in which polymer molecules are arranged in matrix, a clad composed of organic materials having different refractivity from the core. (The clad usually has a low refractive index). Especially, there is a plastic optical fiber of refractive index distribution type in which the core has refractive index distribution from the center toward the outside. In this structure, the band for transmission of the optical signal is able to be made larger. Therefore, this type of the plastic optical fiber has high transmission capacity and attracts attention to use for the high transmission. Several methods of producing the optical member of the refractive index distribution type are proposed, and for example in Japanese Patent Publication No. 3332922, there is a method in which interfacial gel polymerization is made to form a preform, and the preform is drawn to produce the graded index plastic optical fiber (hereinafter, GI-POF).
In the interfacial gel polymerization, polymerizable compounds are supplied into an inside of a tube so as to melt or swell an inner surface of the tube. In this situation, the interface between the polymerizable compounds and the inner wall of the tube swells to have a swollen phase. Then the polymerization proceeds in the gel effects from the inner wall gradually. Thereby when the polymerizable compounds contain a dopant having a large molecular volume, the content of the dopant in the central portion becomes large. Thus the preform is produced so as to have a portion having a continuous refractive index distribution in section.
The preform having the above refractive index distribution can be used as a perform for other optical members (for example GRIN (Graded-Index) lens) if the drawing is not made but the processing, such as the cutting. Therefore the serviceability of the preform is large.
In a method of forming the preform, for example, a clad tube is prepared, and polymerizable monomers (including pre-polymerized compounds) are supplied into an inner space of the clad tube for forming a light guide portion. In this case, there are some combinations of the clad and the core, which makes a clad/core interface irregularity and the lower performance of the preform. Therefore an intermittent layer is provided.
FIGS. 14A-14D illustrate the procedure of forming the preform provided with the intermittent layer. A clad tube 150 is formed of polymer and has a length L11, and polymerizable compounds (outer core liquid) whose main component is the polymerizable monomers (including the pre-polymerized compounds) are supplied in an inner space of the clad tube 150. Then the polymerization is made with rotation of the clad tube 150 (this polymerization is called rotating polymerization), so as to form in the clad tube 150 an outer core portion 152 made of the polymers of the polymerizable monomers. The outer core portion 152 has an inner space of a diameter D11, and becomes an outer core of the POF. Further, the clad tube 150 is provided with a stopper 151 to stop an end thereof, and then polymerizable compounds as an inner core liquid 153 for forming an inner core is supplied into the inner space of the outer core portion 152. The inner core liquid 153 contains monomers as a main content, polymerization initiator, chain transfer agent, refractive index modifier and the like. When the clad tube 150 is heated or disposed in an illumination of light, the polymerization of the monomers in the inner core liquid 153 starts. Thus a inner core portion whose main content is polymer is formed. As shown in FIG. 14B, in the polymerization of the monomers, the contraction in volume of the inner core liquid 153 occurs, and the solvent in the inner core liquids 153 is evaporated to be gasses 154. Thereby parts of the generated gasses 154 (gas compounds) are remaining dissolved in the inner core liquids 153. Then in the procedure of the polymerization with decreasing the volumes, as shown in FIG. 14C, an upper surface of the inner core liquid 153 forms a V-shaped retraction (or V-contraction). Then, as shown in FIG. 14D, the polymerization of the inner core liquid 153 is complete to form the inner core portion 155. Thus the preform 156 is obtained, and the inner core portion 155 forms an inner core of the POF.
However, in the methods in FIGS. 14A-14D, the contraction in volume occurs in the polymerization. In this case, it is necessary to determine a thickness of the outer core portion 152 formed on an inner face of the clad tube 150, an outer diameter and a thickness of the clad tube 150, such that the diameter D11 may have enough largeness for forming the inner core portion 155. If the diameter D11 is too small, the evaporation for generating the gasses 154 is not enough made, and the formed preform 156 sometimes contains the gas compounds of the gasses 154 in the inner core 155. If the diameter D11 is too large, the evaporation is not made from a liquid surface enough, the polymerization reaction of the inner core liquid 153 cannot be made uniformly. Otherwise, the viscosity of the inner core liquid 153 increases in the polymerization. In this case, if the length L11 of the clad pipe 150 is too large, the gasses 154 generated in a lower side is contained in and cannot escape from the inner core liquid 153. Thus the inner core portion 155 is formed with containing the gasses 154 to causes voids therein.
Further, the preform is drawn with melting to form the POF. Thereby, the voids are generated in the melt polymer so as to remain in the POF as the products, even though the voids are not visible after the polymerization to form the inner core portion 155. At the portion of the void, the optical transmission properties become worse, and therefore the productivity becomes lower.
One of characteristics of the plastic optical fiber (hereinafter POF) is a larger diameter than the quartz optical fiber as above, and the POF can be easily connected to a light source, such as a light emitting diode (LED), a laser diode (LD), a vertical cavity surface emitting laser (VCSEL) and the like. Recently, the production of the POF of the refractive index distribution type became possible, and it is required to use the POF of this type as a transmission path of a transmission link for transmitting optical signals in a broadband of at least 1 Gbps (100 m). In the transmission link (1 Gbps (100 m)) of this case, the superiority in the optical coupling of the light source to the POF of the large diameter is the same as to the quartz optical fiber. However, in the optical coupling of the POF of the large diameter to a light receiving device (for example photodiode) for receiving an exit light from the POF, in order to make the good optical coupling for the transmission link (1 Gbps (100 m)), a condensing device (such as an optical lens and the like) is disposed in a space between the POF and the light receiving device. The reason therefor is that the exit light from the POF of the large diameter extends in effect of the diffraction to more than a diameter of an aperture of a light receiving device.
Also a connector for connecting the POFs in the transmission link of optical signals has the same problem. The connector for the POFs is designed, in consideration of elongation of the POFs in the change of the temperature, such that there may be a space about few hundreds micrometers between the connected POFs. Therefore, the exit light from the POF extends in the diffraction to have a beam diameter equal to or more than a fiber diameter (or aperture diameter) of the next POF. Thus when the exit light enters into the next POF, the optical coupling loss (diffraction loss) occurs. Accordingly, in order to make the optical coupling of good efficiency also in the connector for the POFs, it is necessary to dispose the condensing device (relay lens and the like) in the connector. In the case of connecting the quarts fibers, their ends are physically contacted to each other, and therefore, there are no problems as described above.
Further, in the transmission link for higher-bandwidth with use of the POFs, attentions to a system for multi-wavelength optical transmission became larger, in which it is necessary to dispose an optically functional element (such as a wavelength filter and the like) between the transmission paths for separating the signals of multiplied wavelength. However, if the space for disposing the optically functional element without condensing function on the transmission path, the diffraction loss occurs and a coupling efficiency of the optical coupling becomes lower. In order to prevent the decrease of the coupling efficiency, an optical collimator constructed of lenses is disposed between the transmission path.
As described above, the condensing device is usually used in the transmission link of optical signal with use of the POF, in order to decrease the diffraction loss, or to improve the coupling efficiency. Further, the condensing device is more important when the quartz optical fiber is used than when the POF is used as the transmission path of the transmitting link for optical signal. When the POF is used, therefore, in order that the light receiving device effectively receives the optical power, the structure, design and the like of the condensing device is more important than when the quartz optical fiber is used.
In the prior art, as the condensing device disposed between the optical fiber and the light receiving device, there are concretely a lens described in Japanese Patent Laid-Open Publication No. 2000-147294, a graded-index lens (or GRIN lens) described in Japanese Patent Laid-Open Publication No. 2001-264592, and the like. Further, in a condensing device of Japanese Patent Laid-Open Publications No. 8-21929 & 8-75935, an end of the POF may be processed to have a curved surface (such as spherical surface) or a taper-form. In these cases, since the condensing of the emitted light from the POF can be made, the focused light can be received by the light receiving device, and thus the optical coupling efficiency becomes larger.
However, in the methods of the above four publications, it is necessary to accurately decide a position of the condensing device (for example, condensing length of the lens and the like) which condenses a transmission light to an emission axis of the optical fiber. Accordingly, the number of parts for the optical coupling is increased, and the alignment between the POF, the optical lens and the light receiving device is necessary. Therefore the cost of the link system for optical transmission becomes higher.
Further, if there is an optical element such as a lens between the POF and the light receiving element, the cost becomes higher and a Fresnel loss occurs such that the power of the transmission light becomes lower. Accordingly, it is necessary to make the optical coupling in which the smaller number of the optical elements is disposed. Further, the exit end face may contact directly to light receiving element. However, in this case, it is hard to make the optical coupling in the simple structure, since the light receiving elements or the exit end face thereof is broken by forcedly pressing to the light receiving element. Therefore, it is necessary to design the positional relation between the exit end and the receiving device and to fix them concretely, and the easy optical coupling cannot be made.